Re: [RFC PATCH v4 3/9] x86/cet/ibt: Add IBT legacy code bitmap allocation function

[Andy Lutomirski]

> On Oct 5, 2018, at 9:13 AM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> 
>> On Wed, 2018-10-03 at 21:57 +0200, Eugene Syromiatnikov wrote:
>>> On Fri, Sep 21, 2018 at 08:05:47AM -0700, Yu-cheng Yu wrote:
>>> Indirect branch tracking provides an optional legacy code bitmap
>>> that indicates locations of non-IBT compatible code.  When set,
>>> each bit in the bitmap represents a page in the linear address is
>>> legacy code.
>>> 
>>> We allocate the bitmap only when the application requests it.
>>> Most applications do not need the bitmap.
>>> 
>>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
>>> ---
>>> arch/x86/kernel/cet.c | 45 +++++++++++++++++++++++++++++++++++++++++++
>>> 1 file changed, 45 insertions(+)
>>> 
>>> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
>>> index 6adfe795d692..a65d9745af08 100644
>>> --- a/arch/x86/kernel/cet.c
>>> +++ b/arch/x86/kernel/cet.c
>>> @@ -314,3 +314,48 @@ void cet_disable_ibt(void)
>>>    wrmsrl(MSR_IA32_U_CET, r);
>>>    current->thread.cet.ibt_enabled = 0;
>>> }
>>> +
>>> +int cet_setup_ibt_bitmap(void)
>>> +{
>>> +    u64 r;
>>> +    unsigned long bitmap;
>>> +    unsigned long size;
>>> +
>>> +    if (!cpu_feature_enabled(X86_FEATURE_IBT))
>>> +        return -EOPNOTSUPP;
>>> +
>>> +    if (!current->thread.cet.ibt_bitmap_addr) {
>>> +        /*
>>> +         * Calculate size and put in thread header.
>>> +         * may_expand_vm() needs this information.
>>> +         */
>>> +        size = TASK_SIZE / PAGE_SIZE / BITS_PER_BYTE;
>> 
>> TASK_SIZE_MAX is likely needed here, as an application can easily switch
>> between long an 32-bit protected mode.  And then the case of a CPU that
>> doesn't support 5LPT.
> 
> If we had calculated bitmap size from TASK_SIZE_MAX, all 32-bit apps would have
> failed the allocation for bitmap size > TASK_SIZE.  Please see values below,
> which is printed from the current code.
> 
> Yu-cheng
> 
> 
> x64:
> TASK_SIZE_MAX    = 0000 7fff ffff f000
> TASK_SIZE    = 0000 7fff ffff f000
> bitmap size    = 0000 0000 ffff ffff
> 
> x32:
> TASK_SIZE_MAX    = 0000 7fff ffff f000
> TASK_SIZE    = 0000 0000 ffff e000
> bitmap size    = 0000 0000 0001 ffff
> 

I havenât followed all the details here, but I have a general policy of objecting to any new use of TASK_SIZE. If you really really need to depend on 32-bitness in new code, please figure out what exactly you mean by â32-bitâ and use an explicit check.

Some day I would love to delete TASK_SIZE.